Education and Research in Engineering Assessment and Rehabilitation of Historic Wood and Masonry Buildings E.U. Perspective

Transcription

1 Education and Research in Engineering Assessment and Rehabilitation of Historic Wood and Masonry Buildings E.U. Perspective Clara Bertolini Cestari Alberto Carpinteri, Stefano Invernizzi, Giuseppe Lacidogna Abstract The preservation, maintenance and restoration of the wood and masonry architectural heritage is traditionally taught in European Community Faculties of Architecture and Civil Engineering, in the context of MS BC, Master and PhD courses especially devoted to the issue. At the beginning will be described the sector of historic masonry construction and subsequently the one of wood. Clara Bertolini Cestari Professor of Technology and Construction of Architecture Dipartimento di Progettazione Architettonica e di Disegno Industriale Politecnico di Torino, Torino, Italy. Alberto Carpinteri Chair of Structural Mechanics Dipartimento di Ingegneria Strutturale e Geotecnica Politecnico di Torino, Torino, Italy. Stefano Invernizzi Assistant Professor Dipartimento di Ingegneria Strutturale e Geotecnica Politecnico di Torino, Torino, Italy. Giuseppe Lacidogna Assistant Professor Dipartimento di Ingegneria Strutturale e Geotecnica Politecnico di Torino, Torino, Italy. 1. Introduction Assessing the safety of the large historical buildings and masonry towers that characterize the Italian cultural heritage is a complex problem, which must be addressed by interpreting their environmental and loading conditions and the characteristics of the constituent materials [1-5]. The Italian schools are develop the majority of didactic and research activities on the theme of cultural heritage, compared to the other European countries. The accumulation of damages, the presence of cracks and time-dependent phenomena such as creep are decisive factors in determining the life-time these structures. Moreover, in order to be able to formulate predictions on their stability, it is necessary to be able to identify damage signals and to determine their causes. In this connection, ever greater importance is attached to the adoption of innovative, highly advanced monitoring techniques for the analyses of such phenomena [6]. In many instances, the causes and the effects of damage can be correlated only by defining appropriate analytical modelling parameters through experimental investigations [7]. Creating an analytical or a numerical model of a masonry structure is no simple task: masonry, in fact, seems to elude all the assumptions made for other structural materials, such as isotropy, homogeneity and elastic behaviour, and the constitutive laws for this material have not been fully developed yet. In this connection, interesting numerical and experimental studies on historic buildings have been conducted recently with a view to addressing the main non-linear aspects of structural behaviour [8,9]. Moreover, in terms of advanced monitoring techniques, a promising method using a non destructive technique based

2 on the spontaneous emission of pressure waves from cracking processes underway has been developed in recent years [10]. This method, referred to as Acoustic Emission (AE) technique and originally applied to industrial steel structures (such as pressurised chemical or nuclear vessels), has now found application in the control of civil structures [11-14]. Having identified the location of cracks or other lesions in a structure, it is possible to evaluate the evolution of the damage, which may come gradually to a halt or spread at a faster rate. With this technique, by means of a plurality of sensors and through triangulation, it is also possible to pinpoint the position of damages (not known beforehand) to be able to arrive at an assessment of their stability [15]. Several analyses were conducted with this technique on historic buildings and masonry towers, some of them rising in seismic zones, in order to estimate the amount of energy released during cracking and damaging processes and thereby gather information on the criticality of the damaging processes underway [16,17]. In particular, from the monitoring of medieval masonry towers it proved possible to determine that acoustic emissions from a structure can be correlated, through Richter's laws, with local seismic activities [18,19]. Another fundamental line of research is the identification of appropriate methods for structural rehabilitation. In this connection, laboratory tests were conducted to assess the shear strength of masonry wall consisting of several layers. These tests were performed with the simultaneous application of the AE technique in order to evaluate the effectiveness of repair methods by means of special mortars or metal braces [20]. Moreover, the use of fibre-reinforced composites with a polymeric matrix (Fibre Reinforced Polymers - FRP) in structural restoration works has been gaining ground and acquiring a predominant role [21,22]. In fact, the use of this technology can meet several essential architectural requirements associated with the rehabilitation of historical buildings, which cannot be fulfilled by traditional techniques, e.g., removability, absence of water and vibrations in the restoration processes, respect for pre-existing elements (such as frescoes on vaults)[23-25]. The Research Unit of the Turin Politecnico developed and tested an original methodology for the localisation of acoustic emissions in beams in bending that had been strengthened through the external application of carbon fibres (FRP) and tested till failure [26]. A similar testing campaign is underway on masonry elements strengthened through the same procedure and subject to shear. The testing campaign has been designed to assess, with the aid of numerical methods, the efficiency of these strengthening methods by identifying the sources of damage, i.e., the zones where the detachment starts and hence the process leading to complete delamination takes place through snap-back type instability processes [27]. 2. Current educational E.U. programs In general, Italian universities in the MS and BC degree courses, since ten years ago have started international exchange programmes with the other European Universities, in the frame of the Erasmus/Socrates programme. This European programme contains a wide range of measures designed to support the European activities of higher education institutions and to promote the mobility and exchange of their teaching staff and students. Higher education plays a crucial role in producing high quality human resources, disseminating scientific discovery and advanced knowledge through teaching, adapting to the constantly emerging needs for new competences and qualifications, and educating future generations of citizens in a European context. All such functions are of vital importance to the long-term development of Europe. While the promotion of 'physical mobility', mainly of students, constituted the main thrust of ERASMUS Phase I and II, the higher education Chapter of SOCRATES seeks to integrate such mobility into a wider framework of cooperation activities which aim at developing a "European Dimension" within the entire range of a university's academic programmes. "Bringing students to Europe, bringing Europe to all students" is the new spirit of ERASMUS: while student mobility retains a position of central importance within the programme, stronger

3 incentives are now available to encourage universities to add a European perspective to the courses followed by students who do not participate directly in mobility. More emphasis is consequently placed on teaching staff exchanges, transnational curriculum development and pan-european thematic networks. Wider dissemination of and participation in the results of this work are sought through specific support. ERASMUS also encourages universities to associate other public and private bodies from their surrounding regions with their transnational cooperation activities, thereby enhancing opportunities for inter-regional cooperation between the participating countries. In the Incoming activity, both of students, both of teaching staff, the exchanges towards Italy are required especially for the disciplines addressed towards cultural heritage. Among those disciplines there is in-situ evaluation of wood and masonry buildings. For example the Faculty of Architecture of Politecnico di Torino has several exchanges in the frame of Erasmus Programme with European universities of high education [28]. More recently, since the beginning of 2007, has been established a new postgraduate Master s Program co-ordinated by the Vienna University of Technology: Urban Wood Wood Based Building Design for Sustainable Urban Development [29]. For this international MSc Program, students share their time between the three participating universities in Dresden, Turin and Vienna. Pertinent study trips and a clear thematic structure for the three places of study in combination with an international faculty will provide the postgraduate students with a forward-looking approach to building with wood at many different levels. Sustainability is a central issue. The increasing shortage of resources and the climatic changes require significant reductions in the level of energy consumption. Wood is a renewable resource and very suitable for environmentally compatible construction also in an urban context. Current and future wood-based construction methods are

4 always the result of complex technological challenges and engineering excellence. Highperformance combinations of materials and intelligent pre-fabrication methods result in new ways of looking at wood as a building material. The wide range of challenges designers are confronted with requires comprehensive expertise and an extensive array of innovative solutions. To facilitate such solutions, Vienna University of Technology, Dresden University of Technology and Politecnico di Torino offer this postgraduate Master s Program which combines international mobility with comprehensive, high-level teaching. The MSc Program Urban Wood provides participants with a solid knowledge base for research, as well as the necessary skills to manage the design and construction of wood-based building projects in an urban context. The first year of the program is divided into three modules in accordance with each institution's specific field of expertise. In Dresden, students will focus on a general study of wood-based construction systems and acquire knowledge on wood-based materials. The Politecnico in Turin places the focus on the Restoration and refurbishment of historical wood-based buildings [30]. In Vienna, participants will learn about wood-based multistory buildings in an urban context. manage the design and construction of woodbased building projects. It also educates them to better exploit the adaptable qualities of wood as a result of improved technological and economical mastery of the material. Yet the postgraduate course, which is mainly focused on the technical side of the art of timber construction, does not neglect the interdependence between architectural, technical and socio-economical aspects. Alumni of the Master Program are expected to have acquired the skills needed to direct the design and construction of major timber buildings in an urban context. Finally, the development of timber building systems and wood-based materials on the basis of such proficiency must be regarded as a valuable supplementary qualification. Organisation plan of the International Urban Wood post-graduate Master Course: open to the worldwide scientific and educational community. The image refers to the timber roof-structure of the Valentino Castle in Torino, XVII th century. 3. Opportunities As example of post-graduate degree course for architects and engineers is the one of MSc Program Urban Wood that provides the participants with basic scientific knowledge for research, as well as the necessary skills to 4. New trends in the research 4.1 In general For the safeguard of the monuments and the historic building heritage of the Italian territory it is increasingly important to make use of advanced, non-destructive monitoring techniques. Among such methods, the acoustic emission technique is the most promising, since it is the only one that can identify a damaging process the very moment it occurs. The deformation and cracking processes that take place in materials subject to states of stress, in fact, give rise to the release of energy in the

5 form of ultrasound vibrations and pressure waves that propagate through the material and can be received and recorded by sensors applied to the external surfaces of a structure. Acoustic emissions are generally detected by means of piezoelectric transducers (PZT) that exploit the ability of some crystals to generate electrical signals whenever they are subjected to mechanical stresses. Resonant or wide band transducers can both be used, provided that they are calibrated on frequency values of between 50 and 800 khz. The acquisition of the ultrasound signals due to cracking, mostly the longitudinal waves (p-waves), is by means of transducers connected to control units specially programmed to process such signals. Moreover, the control units can be integrated with a system for the telematic acquisition of the data. This process is similar to the operations that are performed on a much larger scale in seismology, where shock waves are perceived by stations installed at different points on the surface of the Earth. The intention is to use the AE technique to fine-tune an investigation method for the estimation of defects caused by mechanical damaging in historic buildings and masonry towers. A very delicate stage of the research program, to be performed in the laboratory, will be the study of micro-structural failure mechanisms in concrete and masonry, to be used in the analysis of the mechanical properties of the materials and in the development of rational constitutive models. Non-linearity and strain-softening phenomena will be incorporated into the models used for predicting the behaviour of the materials. Moreover, specific analyses will be conducted to investigate fatigue behaviour and creep effects. Tests will be conducted in parallel by the Milan Unit for the calibration of the new masonry creep model that they want to implement. Furthermore, the study will focus on methods geared to the localisation and quantification of the sources of acoustic emissions in structures in serviceability conditions. Another goal envisaged is to determine on a larger scale - provided that the transmission speeds of the pressure waves and the differences between their arrival times to the various transducers are known - the zones of origin, the orientation and the causes of the cracks. This highly advanced procedure, called Moment Tensor Analysis (MTA), is also used in seismology, where the seismic moment is a parameter directly correlated with fracture problems in the Earth's crust. The Research Unit of the Politecnico di Torino has already acquired extensive experience in the monitoring of historic buildings with the AE technique. Among the most mportant buildings studied with this method we should mention the medieval towers of the City of Alba, or the bearing structures of the Temple of Athena incorporated into the masonry walls of the Cathedral of Siracusa (investigation carried out in collaboration with the Politecnico di Milano). It is believed that in the most critical cases, or those requiring prolonged observation periods, the AE monitoring method can be adjusted for the use of a telematic working procedure that does away with the need of having the operators work on site and makes it possible to receive and process the signals from a distance. Thus, it will be possible to use a centralised station to control continuously and simultaneously, in real time, individual structural elements or entire structures, possibly situated in different places. The results obtained with this method on actual structures may then be compared with the results of the Radar method used by the Milan Unit and applied to the same structures at different times. Moreover, due to the heterogeneity of the constituent units of concrete and masonry structures, the US signals generated locally by the damaging process are subject to considerable attenuation and dispersion phenomena. A valid control of the structures would require the application of a great number of sensors to cover all the critical zones. On the other hand, the classical connections via cables between the sensors and the data processing units would result in a very inefficient set-up, entailing high installation costs and making the AE monitoring technique costly and invasive. Thus, it is believed that huge structures should be monitored by means of sensors of a new type, using electronic wireless systems and efficient algorithms for processing large quantities of data. Wireless sensors could be used in a Local Area Network system (LAN), based on highfrequency radio transmissions, thereby avoiding

6 the use of traditional cables. Their use would be cheaper and more flexible since, if the need arose, it could be supplemented with traditional cable networks. As is known, in non homogeneous or composite materials, such as masonry, the energy dissipated during crack growth plays a fundamental role in the definition of the residual life of the structures. Laboratory research activities simulating damage processes on full scale structures have included a series of compressive tests on masonry specimens, performed with an MTS interfaced with an AE monitoring system. Similar tests, on masonry test pieces of different sizes, will also be performed in situ through the simultaneous application of the double flat jacks and the AE techniques. Such tests will be supplemented by triaxial tests on smaller-sized test pieces conducted at the Politecnico di Milano. A series of monoaxial compression tests performed by the Research Unit of the Turin Politecnico demonstrated appreciable size effects on the density of the energy dissipated during the application of compressive loads. Based on these studies, a fractal theory that makes it possible to analyse and monitor stone or brick masonry structures with the AE technique has been developed. This fractal theory takes into account the multi-scale nature of the energy dissipated. In this manner it proved possible to introduce an appropriate energy parameter for the evaluation of structural damage. This parameter is based on the correlation between the cumulative AE activity recorded in a structure and the corresponding AE events recorded in test pieces obtained from the structure and tested till failure. With this parameter, by applying both the fractal theory and AE tools it is possible to evaluate the safety and residual life of structures undergoing damaging and deterioration processes. The experimental results will be interpreted by means of a numerical model based on the Finite Element Method, able to reproduce the main mechanical features as the fracture nucleation, the fracture propagation and the time dependency. The main goal will be to link the dissipated energy, acquired from the acoustic emissions (AE), and the fracture localization with the results obtained numerically, e.g. the fracture propagation at a certain Gauss point of a finite element when a smeared crack constitutive model is adopted. In this way, a quantitative tool will be obtained for the interpretation of the experimental results, which is also exploitable in case of complex structural geometry. First of all, this tool will be applied to the simulation and interpretation of laboratory tests. Subsequently, more complex in situ tests will be considered, such as, for instance, tests by means of flat jacks. In addition, a rheological numerical damage model will be developed in order to understand the fracture propagation and acoustic phenomena not only as a function of increasing loads, but also for increasing time being the load constant. This model will help with the interpretation of the acoustic emissions when assessing the residual service life of the structure, allowing for a time dependent evaluation of the safety factor, and providing information about the urgency of the retrofitting intervention. The above numerical model will be validated and used to simulate the mechanical response of historical large masonry buildings and towers. The analysis will be performed on several case studies, in order to develop a general procedure. The AE technique is a dependable method for assessing the efficiency of the different structural upgrading and strengthening techniques developed for r.c. and masonry structures, such as plating with fibrereinforced composites (FRP). In fact, despite the widespread utilisation of these interventions, some of the aspects associated with their durability and reliability are not very clear yet. In these cases, AE monitoring can be performed before and after the intervention, in order to evaluate the variation in acoustic emissions brought about by the rehabilitation works. To this end, it is proposed to perform a laboratory testing campaign on masonry specimens, made with stone or bricks and mortar, reinforced with strips of carbon fibre sheets, in order to verify durability and resistance to compressive and/or shear stresses. These tests will be performed in liaison with the Padua Unit, which will also study alternative strengthening methods. Structural models will have to be produced on different scales to study size effects on the strength of the materials. The tests should be performed first of all by applying static loads to

7 the structures in order to identify damage processes, the increase in ductility and the transition to collapse by delamination. Of special interest will also be the assessment of the behaviour of structural elements under fatigue and creep effects. The action of repeated loading might trigger particularly critical delamination processes in brittle materials, such as brickwork, having low tensile strength and stiffness values varying as a function of the load applied. Moreover, polymeric matrix materials are seen to undergo not negligible time-dependent rheological phenomena, which may even undermine their mechanical properties. During load tests, the structural models will be monitored by means of advanced AE localisation systems, so as to characterise the delamination or cracking process as a function of the size of the elements analysed. The validity of instrumented monitoring results will be assessed by making use of appropriate analytical and numerical models to interpret and predict the experimental findings. 4.2 Wooden structures Conservation and maintenance represent strategic fields of interest in EU for their high social/economic impact. Timber structures may present degradation phenomena, even catastrophic as in recent disasters in EU. An integrated approach for interventions on timber structures is essential for the project, by considering use of materials, techniques, devices with high technological content and by taking into account different aspects of optimisation, innovation and improvement of existing methods. The complete process is considered as a whole, not only as related to the single phase of reinforcing/restoration but also regarding maintenance (including monitoring and retrofitting) of structures, so to guarantee safety with a general cost saving approach. Main objectives of the innovative research are: -considering new adhesive formulations and techniques, applicable on site, able to efficiently reinforce timber. Nanomaterials (e.g. carbon nanotubes), thanks to their properties, are compatible with wood structure; -improvement/tailoring of devices to automatically collect, transmit, store information and to process it, exploiting conductive properties of nanotube additivated resins; -definition of innovative maintenance, based on continuous and automatic monitoring without the need of expensive periodical inspections. Savings and improvement of safety against unexpected failures are expected; -modernisation of traditional techniques of SMEs with innovative products and services for end-users, respecting all environmental, safety and health requirements [31]. 5. Conclusions From this brief overview on the themes of education and research it appears evident the necessity to integrate competences and experiences in the sector of NDT diagnosis not only at European level, but extending the field also to new networks proposing interdisciplinary, multidisciplinary and innovative projects of research and development. In our opinion, Rilem could be an important cluster. 6. References [1] G. Croci: "Progettazione Strutturale e Consolidamento delle Costruzioni", Ed. Hoepli, Milano (1982). [2] A. Carpinteri, P. Bocca (Editors): "Damage and Diagnosis of Materials and Structures", Pitagora Editrice, Bologna, (1991). [3] L. Binda, A. Saisi, C. Tiraboschi: "Investigation procedures for the diagnosis of historic masonries", Construction and Building Materials, Vol. 14 (2000), [4] A. Anzani, L. Binda, G. Mirabella Roberti: "The effect of heavy persistent actions into the behaviour of ancient masonry", Materials and Structures, Vol. 33 (2000), [5] C. Modena, P.B. Lourenco, P. Roca (Editors): "Structural Analysis of Historical Constructions: Possibilities of numerical and experimental techniques", A. A. Balkema Publishers, Leiden, (2004). [6] A. Carpinteri, G. Lacidogna, N. Pugno: "Structural damage diagnosis and life-time assessment by acoustic emission monitoring", Engineering Fracture Mechanics, Vol. 74 (2007), [7] A. Carpinteri, S. Invernizzi, G. Lacidogna: "In situ damage assessment and nonlinear modelling

8 of an historical masonry tower", Engineering Structures, Vol. 27 (2005), [8] A. Carpinteri, S. Invernizzi, G. Lacidogna: "Structural assessment of a XVIIth century masonry vault with AE and numerical techniques", International Journal of Architectural Heritage, Vol. 1 (2007), [9] A. Carpinteri, S. Invernizzi, G. Lacidogna: "Numerical assessment of three medieval masonry towers subjected to different loading conditions", Masonry International, Vol. 19 (2006), [10] A. Carpinteri, G. Lacidogna, N. Pugno: "A fractal approach for damage detection in concrete and masonry structures by the acoustic emission technique", Acoustique et Tecniques, Vol. 38 (2004), [11] M. Ohtsu: "The history and development of acoustic emission in concrete engineering", Magazine of Concrete Research, Vol. 48 (1996), [12] M. Ohtsu, T. Okamoto, S. Yuyama: "Moment tensor analysis of acoustic emission for cracking mechanisms in concrete", ACI Structural Journal, Vol. 95, (1998), [13] C.U. Grosse, H.W. Reinhardt, F. Finck: "Signalbased acoustic emission techniques in civil engineering", ASCE Journal of Materials in Civil Engineering, Vol. 15 (2003), [14] S. Colombo, I.G. Main, M.C. Forde: "Assessing damage of reinforced concrete beam using b- value analysis of acoustic emission signals", ASCE Journal of Materials in Civil Engineering, Vol. 15 (2003), [15] A. Carpinteri, G. Lacidogna, G. Niccolini: "Critical behaviour in concrete structures and damage localization by acoustic emission", Key Engineering Materials, Vol. 312 (2006), [16] A. Carpinteri, G. Lacidogna: "Damage monitoring of an historical masonry building by the acoustic emission technique", Materials and Structures, Vol. 39 (2006), [17] A. Carpinteri, G. Lacidogna: "Structural monitoring and integrity assessment of medieval towers", ASCE Journal of Structural Engineering, Vol. 132 (2006), [18] C.F. Richter: "Elementary Seismology", W. H. Freeman and Company, San Francisco and London (1958). [19] A. Carpinteri, G. Lacidogna, G. Niccolini: "Acoustic emission monitoring of medieval towers considered as sensitive earthquake receptors", Natural Hazards and Earth System Sciences, Vol. 7 (2007), [20] A. Anzani, L. Binda, A. Carpinteri, G. Lacidogna, A. Manuello: "Repair of damaged multiple leaf masonry: assessment by the Acoustic Emission technique", in Fracture Mechanics of Concrete Structures (Proceedings of the 6th International FraMCoS-6 Conference), Eds. A. Carpinteri et al., Taylor & Francis, London (2007), Vol. 3, [21] AA.VV.: "Istruzioni per la progettazione, l'esecuzione ed il controllo di interventi di consolidamento statico mediante l'utilizzo di compositi fibrorinforzati". CNR-DT200/2004. [22] fib Bulletin: "Design and Use of Externally Bonded FRP Reinforcement (FRP EBR) for Reinforced Concrete Structures", Bulletin no. 14, prepared by sub-group EBR (Externally Bonded Reinforcement) of fib Task Group 9.3 FRP Reinforcement for Concrete Structures (2001). [23] M. Arduini, A. Di Tommaso, A. Nanni: "Brittle failure in FRP plate and sheet bonded beams", ACI Structural Journal, Vol. 94 (1997), [24] F.J. Alaee, B.L. Karihaloo: "Fracture model for flexural failure of beams retrofitted with CARDIFRC", ASCE Journal of Engineering Mechanics, Vol. 129 (2003), [25] A. Di Tommaso, (Editor): "Mechanics of masonry structures strengthened with FRP materials: modeling, testing, design, control", Second National Congress, Venice, [26] A. Carpinteri, G. Lacidogna, M. Paggi: "Acoustic emission monitoring and numerical modeling of FRP delamination in RC beams with non-rectangular cross-section", Materials and Structures, Vol. 40 (2007), [27] A. Carpinteri: "Cusp catastrophe interpretation of fracture instability", Journal of the Mechanics and Physics of Solids Vol. 37 (1989), [28] List of website links of the main European Universities that have exchanges in the frame of Erasmus Programme with the Faculty of Architecture of Politecnico di Torino:

9 c/default.aspx?culture=en- US&contentlan=2&nodeid= Marseille/ pagelibre/& RH = d= interventions and maintenance, in the frame of the EU 7th Framework Programme. [29] [30] The Co-ordinator of the Module 2 Restoration and refurbishment of historical wood-based buildings of Urbanwood Master, Prof. Clara Bertolini Cestari says: Ancient timber structures: at present, on the subject of their conservation, it is widespread the conviction that those structures must - as far as possible - be preserved, refurbished, restored, in their original static function, with interventions that are respectful and coherent to their original typology, but also to their material: WOOD. With these objectives it is given great importance at the teaching of NDT in-situ inspection, evaluation and survey on ancient timber constructions. [31] These specific themes of research are faced at Italian National level in a research project financed by the Ministry of Scientific Research and at European Level in the proposal TT- TendedTimber. Nanotechnologies/nanosciences for the safety of timber constructions: wood improvement and reinforcement, monitoring of